Free Bus Rides; A Question

[DetroitDad]

So, after taking the 53 and a few other routes over the past couple years, I have found it interesting that about once a week [[using the bus 3-6 days a week, depending on the weather) I do not have to pay for the bus. The driver just waives their hand over the toll box and says to get on, along with everyone else who gets on. This is of course, on the busiest line, and is often during peak times. What is going on?

If I do not buy a pass, and am just taking the single line, this saves me $153 a year. On a full bus of 52 passengers that is $8,112 a year that is lost on the 53 Route, assuming that bus is the only one not accepting fares [[number figured on a round trip, meaning it's actually happening twice a week).

The bus has been completely full before when this has happened.

What's going on?